1. Field of the Invention
The present invention relates in general to an irradiation system used for radiation therapy administering radiation to an irradiation target such as a tumor, and more particularly to an irradiation system and its irradiation target movement monitoring method, and an irradiation target position recognizing method that can administer radiation by precisely recognizing position and direction of an irradiation target without using a positioning frame, while directly monitoring the position and direction of the target being irradiated.
2. Description of Related Art
The radiation therapy is a treatment to reduce or eliminate tumors by intensively administering radiation to the tumors of a patient. To achieve such a radiation therapy, it is necessary to administer radiation to a tumor, a target to be irradiated, intensively and in proper amount. At the same time, it is necessary to decrease radiation exposure to surrounding healthy tissue as much as possible.
Thus, the radiation therapy is carried out such that excessive irradiation to the tumor and its surrounding healthy tissue is suppressed by administering radiation from multiple directions with particular emphasis on the tumor as the irradiation target, so that only the tumor is intensively irradiated.
To achieve effective radiation only to the target intensively and precisely, it is essential to measure the position and direction of the target, and to decide the pros and cons of the irradiation by computing the relative position and direction between the target and a irradiation head from the computed results.
More specifically, a set of image data of an irradiation region including the target tumor is captured before irradiation, and the relative position and direction between the target and the irradiation head is measured from the set of image data, thereby planning the radiation therapy treatment in terms of the directions and irradiation dose along with the duration and the number of times of the irradiation to implement the radiation therapy according to the plan.
In such a radiation therapy as described above, there is a method to bury a marker near the target tumor, as one of the techniques to measure the position and direction of the target accurately. In this method, fixing the relative position and direction between the marker and irradiation head will enable the position and direction between the target and irradiation head to be measured with respect to the marker regardless of the variations in the position and direction of the target due to movement of the patient, and hence allows the radiation with fixing the relative position and direction between the target and irradiation head.
This method, however, is greatly invasive to the patient in that the marker is buried into the body. Thus, it is preferable to avoid this method as much as possible considering the stress imposed on the patient.
In view of this, another radiotherapy is proposed that can accurately measure the position and direction of the target non-invasively without setting the marker in the body of the patient. This method fixes a solid frame made of metal or the like on the surface of the head of the patient to precisely measure the position and direction between the target and irradiation head with respect to the frame, and administers radiation with fixing the relative position and direction between the target and the irradiation head.
This method utilizes the fact that there is little movement due to respiration or the like within the head because the brain is placed in the skull. Fixing the frame on the surface of the head and maintaining the fixed position can almost fix the relative position and direction between the frame and the target throughout the radiation. Furthermore, the relative position and direction between the frame and the irradiation device (irradiation head) can be accurately measured by fixing the frame on a treatment table or by installing a position and direction sensor on the frame.
Thus, the precise measurement of the position and direction of the frame enables precise position recognition and irradiation of the target tumor.
However, since the frame is usually fixed on the patient from the time of image measurement of the irradiation region including the target tumor for planning the radiation therapy to the time of administering the actual irradiation, the stress imposed on the patient is considerable. In addition, since the frame is fixed rather strong to prevent the frame from shifting, it is unavoidable that the patient has feelings of restraint and pressure. Thus, position recognition of the target tumor without using the frame, if implemented, will serve to improve the therapeutic environment of the patient.
As techniques of implementing the non-invasive radiation therapy of a head region, there are xe2x80x9cRepeat fixation for frameless stereotactic procedurexe2x80x9d disclosed in U.S. Pat. No. 5,588,430, and a radiocamera system of Kobayashi Sofamor Danek utilizing this patent.
FIG. 5 schematically shows a configuration for recognizing the position of an irradiation target in the conventional irradiation system, the xe2x80x9cRepeat fixation for frameless stereotactic procedurexe2x80x9d disclosed in the foregoing U.S. Pat. No. 5,588,430. In this figure, the reference numeral 51 designates a patient""s head including an irradiation target tumor; 52 designates a treatment table on which the patient lies down; and 53 designates a head ring with a structure simpler than the conventional frame, for securing the patient""s head 51 to the treatment table 52. The reference numeral 54 designates a bite plate having LEDs (Light Emitting Diodes) mounted thereon for measuring the position of the body surface of the patient; and 55 designates a mouthpiece that has the bite plate 54 attached thereto, and is bitten by the patient.
Next, the operation of the conventional method will be described.
First, the patient""s head 51 is secured to the treatment table 52 by the head ring 53. Then, the bite plate 54 serving as a marker for measuring the position and direction is attached to the mouthpiece 55 the patient bites. The bite plate 54 is almost immobilized with respect to the patient""s head 51 during the radiation. Accordingly, the relative position and direction between the bite plate 54 and the irradiation target in the head 51 is assumed to be almost invariable. Therefore, measuring the three-dimensional position and direction of the bite plate 54 makes it possible to estimate the position and direction of the target tumor indirectly.
In addition, since the irradiation system allows the patient to be precisely repositioned by only biting the bite plate 54, it is unnecessary for the patient to wear the head ring 53 in the second and the following radiation, thereby reducing the stress on the patient during the therapy.
With the foregoing configuration, the conventional irradiation system is effective for the radiation therapy to a head region. However, it presents a problem in that the irradiation accuracy is degraded in trunk regions such as abdominal organs that are more affected by the body movement like respiration than the head region is, and that are continuously fluctuating the position and direction of the irradiation target.
This problem will be described in more detail.
In the radiation therapy applied to a tumor emerging in a trunk region such as an abdominal organ, since the effect of the body movement like respiration is greater than in the head region, the position and direction of the target tumor continues to vary during the irradiation. Thus, the variations in the position and direction of the irradiation target due to changes in the patient""s posture become greater than those expected in the planning of the radiation therapy. In addition, being different from the head region, the trunk region usually has no bones usable for fixing a frame near the target tumor, which makes it difficult for the frame to be secured on the body surface.
As a result, it is difficult to immobilize the relative position and direction between the target to be irradiated and a jig such as a frame for measuring the position of the body surface.
Besides, to plan a radiation treatment, it is more common to use a set of image data of the irradiation region including the irradiation target acquired before the planning than to use a set of image data taken just before the irradiation.
Thus, during the planning of the radiation therapy, the posture of the patient can vary, and hence the position and direction of the irradiation target. As a result, the irradiation accuracy has some error between the planning of the radiation therapy and the actual irradiation.
Considering the risk rate of bringing about the error of the irradiation accuracy, the actual irradiation is performed assuming some margin around the region including the target tumor. This presents another problem of causing undue radiation exposure to the healthy tissue surrounding the target tumor.
As described above, taking account of the greater error in the irradiation accuracy of the trunk region than that of the head region, the irradiation margin of the trunk region must be set greater than that of the head region. Therefore, the radiation exposure to the healthy tissue surrounding the irradiation target is greater in the trunk region than in the head region.
The conventional irradiation system disclosed in the foregoing U.S. Pat. No. 5,588,430 employs the mouthpiece 55 to which the bite plate 54 is attached, so that it is not necessary to fix a frame to the patient""s head 51 during the period from the image acquisition of the set of image data of the irradiation region including the target to the actual irradiation. However, since the head ring 53, a kind of frame, is fixed on the patient""s head 51, and the mouthpiece 55 is inserted into the mouth, it is unavoidable that the patient has some feelings of restraint and pressure.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide an irradiation system and its irradiation target movement monitoring method, and an irradiation target position recognizing method capable of reducing the stress imposed on the patient, and performing precise radiation therapy not only to the head region, but also to the trunk region without using any positioning frame of the irradiation target.
According to a first aspect of the present invention, there is provided a radiation system comprising: irradiation target positioning means for placing an object having an irradiation target to be subjected to irradiation; irradiation target imaging means for taking images of an irradiation target region including the irradiation target; irradiation means for administering radiation to the irradiation target region according to prescribed irradiation conditions; position and direction measuring means for measuring positions and directions of the irradiation target positioning means, the irradiation target imaging means and the irradiation means, and for computing relative positions and directions between them; irradiation condition correcting means for obtaining position and direction of the irradiation target region in the images using computation results of the position and direction measuring means and compared results obtained by comparing the irradiation target regions in the images successively taken by the irradiation target imaging means, and for correcting the irradiation conditions in which the obtained position and direction is reflected; and control means for controlling the radiation to the irradiation target region in response to the irradiation conditions obtained as a result of the correction by the irradiation condition correcting means.
Here, the position and direction measuring means may measure positions and directions of the irradiation target positioning means, the irradiation target imaging means, and the irradiation means in a 3D coordinate system.
The irradiation target imaging means may comprise a plurality of imaging devices of different modality, and the irradiation condition correcting means may obtain the position and direction of the irradiation target region in the images using computation results obtained by the position and direction measuring means and compared results obtained by comparing the irradiation target region in the images taken by the imaging device of the same modality among the images successively taken by the plurality of imaging devices of different modality.
The irradiation target imaging means may comprise: a high resolution imaging device for taking high resolution 3D images of the irradiation target region which are used for setting the irradiation condition; and a real-time imaging device for taking high resolution 3D images of the irradiation target region before and during irradiation, wherein the irradiation condition correcting means may obtain the position and direction of the irradiation target regions in the images using the computation results obtained by the position and direction measuring means, compared results obtained by comparing the irradiation target regions in the images taken by the high resolution imaging device, and compared results obtained by comparing the irradiation target regions in the images taken by the real-time imaging device, and may correct the irradiation conditions using the images which are acquired by the high resolution imaging device and by the real-time imaging device, and in which the obtained positions and directions are reflected.
According to a second aspect of the present invention, there is provided an irradiation target movement monitoring method of an irradiation system including irradiation target positioning means for placing an object having an irradiation target to be subjected to irradiation, irradiation target imaging means for taking images of an irradiation target region including the irradiation target, and irradiation means for administering radiation to the irradiation target region according to prescribed irradiation conditions, the irradiation target movement monitoring method comprising: an image acquisition step of successively taking images of the irradiation target region by the irradiation target imaging means; a position and direction measuring step of measuring positions and directions of the irradiation target positioning means, the irradiation target imaging means and the irradiation means, and of computing relative positions and directions between them; and an irradiation target monitoring step of obtaining positions and directions of the irradiation target regions in the images using computation results obtained by the position and direction measuring means and compared results obtained by comparing the irradiation target regions in the images successively taken in the image acquisition step.
Here, the irradiation target monitoring step may obtain the positions and directions of the irradiation target regions in the images using computation results obtained in the position and direction measuring step and compared results obtained by comparing the irradiation target regions in the images successively taken by the irradiation target imaging means of the same modality.
According to a third aspect of the present invention, there is provided an irradiation target position recognizing method of an irradiation system including irradiation target positioning means for placing an object having an irradiation target to be subjected to irradiation, irradiation target imaging means for taking images of an irradiation target region including the irradiation target, and irradiation means for administering radiation to the irradiation target region according to prescribed irradiation conditions, the irradiation target movement monitoring method comprising: an image acquisition step of successively taking images of the irradiation target region by the irradiation target imaging means; a position and direction measuring step of measuring positions and directions of the irradiation target positioning means, the irradiation target imaging means and the irradiation means, and of computing relative positions and directions between them; and a target position recognizing step of obtaining positions and directions of the irradiation target regions in the images using computation results obtained by the position and direction measuring means and compared results obtained by comparing the irradiation target regions in the images successively taken in the image acquisition step, and of correcting the irradiation conditions by reflecting the positions and directions in the images.
The target position recognizing step may obtain the positions and directions of the irradiation target regions in the images using computation results obtained in the position and direction measuring step and compared results obtained by comparing the irradiation target regions in the images successively taken by the irradiation target imaging means of the same modality.